Adolf Hitler was gay - or so says a sensational new biography on the Nazi dictator due to be published tomorrow.

Eyewitness accounts from Hitler's former lovers, and historical documents that for the first time illuminate rumours that have circulated for over half a century, are disclosed in Hitler's Secret: The Double Life of a Dictator .

The respected German historian Lothar Machtan even claims in his book that Hitler ordered the deaths of several high-ranking Nazis to prevent the secret of his homosexuality from surfacing.

Ernst Röhm, the leader of Hitler's Sturm Abteilung or Storm Troopers, tried to blackmail Hitler by threatening to reveal his sexuality. Röhm, who was also gay, was murdered as a result, according to Machtan, a history teacher at Bremen University.

He refers to scores of historical documents to support his thesis. In 1915, the young Hitler was a dispatch rider at the front in France. Years later, yet before Hitler became infamous, one of his fellow soldiers, Hans Mend, wrote in his memoirs: 'At night, Hitler lay with Schmidl, his male whore.' Schmidl, otherwise known as Ernst Schmidt, and Hitler were 'inseparable lovers' for five years, according to Machtan.

Hitler's service notes read that as a result of the love affair there was reluctance among senior officers to promote him. According to Erich Ebermeier, a lawyer and writer who viewed Hitler's military files years later: 'Despite his bravery towards the enemy, because of his homosexual activity he lost out on a promotion to non-commissioned officer.'

Police reports from Munich after the First World War also suggest that Hitler was pursued by police because of his sexual orientation. 'As a "brown" [fascist] activist, Hitler managed to lure many young men to his side, but not only for political reasons,' says Machtan.

According to a Munich police protocol from the early part of the 20th century, a 22-year-old man called Joseph told the police: 'I spent the whole night with him.' Another, Michael, who was 18, told them: 'I had been unemployed for months, and my mother and my brother were always hungry, so, at his request, I accompanied the man to his home.' Another, a boy called Franz, said: 'He asked me if I'd like to stay with him and he told me his name was Adolf Hitler.'

The police reports were collected by Otto von Lossow, a German army general who took part in suppressing the Hitler putsch in 1923. He kept the Munich police file for years, as, he described it, 'a form of personal life insurance'. If Hitler had attempted to push him aside, he would have blackmailed him with the information, he said. The police documents were published some years ago in Rome by Eugen Dollmann, a close friend of Heinrich Himmler's and also Hitler's interpreter. But because his book never appeared in German, the startling information remained largely overlooked by historians.

Machtan says that Hitler was particularly drawn to Rudolf Hess, his deputy, who was known in party circles as 'black Emma' and with whom he had spent months in Landsberg prison.

Why, then, did the Nazis persecute homosexuals, sending hundreds of thousands of them to their deaths in labour camps and the gas chambers?

'Hitler himself never condemned homosexuality, but he allowed the persecution of gays in order to disguise his own true colours,' Machtan says.